Pedro Santamaria

A complex genetic locus, polyhomeotic, is required for segmental specification and epidermal development in D. melanogaster

Two mutagenic events are required to make null mutations of polyhomeotic (ph), which suggests that the locus is complex. Amorphic mutations (ph degrees) die in mid-embryogenesis and completely lack ventral thoracic and abdominal epidermal derivatives, whereas single-event mutations lead to transformations similar to those of known dominant gain of function mutants in the Antennapedia and bithorax complexes. After a chromosomal walk, the ph gene was localized using deficiencies and ph mutations that result from DNA rearrangements. Hybridization analyses show that there are two large, duplicated sequences in the ph region, and DNA lesions affecting either one of these repeats alter the function of the ph locus. We propose a model that may account for this unusual functional organization.

Polyhomeotic: A gene of Drosophila melanogaster required for correct expression of segmental identity

SummaryA new locus in Drosophila melanogaster that is required for the correct expression of segmental identity has been discovered. The new locus, termed polyhomeotic (ph), is X-linked and maps cytologically to bands 2D2-3. Homozygous ph flies have homeotic transformations similar to those of known dominant gain of function mutants in the Antennapedia and bithorax complexes (ANT-C, BX-C), and in addition show loss of the humerus. ph interacts with three other similar mutations: Polycomb (Pc), Polycomblike (Pcl), and extra sex comb (esc), and acts as a dominant enhancer of Pc. The expression of ph depends on the ANT-C and BX-C dosage. ph has no embryonic phenotype, but temperature shift studies on ph2 show that the ph+ product is required during embryogenesis and larval development. We propose that ph mutants in some way disrupt the normal expression of the ANT-C and BX-C, and, therefore, that ph+ is needed for maintenance of segmental identity.

Characterization of a region of the X chromosome of Drosophila including multi sex combs (mxc), a Polycomb group gene which also functions as a tumour suppressor

Genetic analysis of the 8D3;8D8-9 segment of the Drosophila melanogaster X chromosome has assigned seven complementation groups to this region, three of which are new. A Polycomb group (Pc-G) gene, multi sex combs (mxc), is characterized and mutant alleles are described. Besides common homeotic transformations characteristic of Pc-G mutants that mimic the ectopic gain of function of BX-C and ANT-C genes, mxc mutants show other phenotypes: they zygotically mimic, in males and females, the characteristic lack of germ line seen in progeny of some maternal effect mutants of the so-called posterior group (the grandchildless phenotype). Loss of normal mxc function can promote uncontrolled malignant growth which indicates a possible relationship between Pc-G genes and tumour suppressor genes. We propose that gain-of-function of genes normally repressed by the wild-type mxc product could, in mxc mutants, give rise to an incoherent signal which would be devoid of meaning in normal development. Such a signal could divert somatic and germ line developmental pathways, provoke the loss of cell affinities, but allow or promote growth.

Proviral amplification of the Gypsy endogenous retrovirus of Drosophila melanogaster involves env-independent invasion of the female germline

Gypsy is an infectious endogenous retrovirus of Drosophila melanogaster. The gypsy proviruses replicate very efficiently in the genome of the progeny of females homozygous for permissive alleles of the flamenco gene. This replicative transposition is correlated with derepression of gypsy expression, specifically in the somatic cells of the ovaries of the permissive mothers. The determinism of this amplification was studied further by making chimeric mothers containing different permissive/restrictive and somatic/germinal lineages. We show here that the derepression of active proviruses in the permissive soma is necessary and sufficient to induce proviral insertions in the progeny, even if the F1 flies derive from restrictive germ cells devoid of active proviruses. Therefore, gypsy endogenous multiplication results from the transfer of some gypsy‐encoded genetic material from the soma towards the germen of the mother and its subsequent insertion into the chromosomes of the progeny. This transfer, however, is not likely to result from retroviral infection of the germline. Indeed, we also show here that the insertion of a tagged gypsy element, mutant for the env gene, occurs at high frequency, independently of the production of gypsy Env proteins by any transcomplementing helper. The possible role of the env gene for horizontal transfer to new hosts is discussed.

polyhomeotic controls engrailed expression and the hedgehog signaling pathway in imaginal discs.

Polycomb group (PcG) genes maintain cell identities during development in insects and mammals and their products are required in many developmental pathways. These include limb morphogenesis in Drosophila melanogaster, since PcG genes interact with identity and pattern specifying genes in imaginal discs and clones of polyhomeotic (ph) null cells induce abnormal limb patterning. Such clones are associated with ectopic expression of engrailed, hedgehog, patched, cubitus interruptus and decapentaplegic, in a compartment specific manner. Our results also reveal negative engrailed regulation by ph in both disc compartments: ph silences engrailed in anterior cells and maintains the level of engrailed expression in posterior ones. We suggest that PcG targets are not exclusively regulated by an on/off mechanism, but that the PcG also exerts negative transcriptional control on active genes.

Partial rescue of dorsal, a maternal effect mutation affecting the dorso-ventral pattern of the Drosophila embryo, by the injection of wild-type cytoplasm.

Mutant alleles at the maternal effect locus dorsal cause a dorsalization of the Drosophila embryo. In extreme mutants, the embryos develop exclusively structures which derive from the dorsal‐most region in normal eggs, in less strong phenotypes in addition to dorsal structures, structures normally derived from a dorso‐lateral to lateral egg region are formed. Injection of cytoplasm from wild‐type embryos into mutant embryos partially restores the dorso‐ventral pattern in that injected embryos develop additional structures never formed in uninjected control embryos or embryos injected with mutant cytoplasm. The phenotype of injected embryos resembles that of weaker alleles at the dorsal locus indicating that the wild‐type cytoplasm partially rescues the mutant phenotype. The response of the mutant embryos is restricted to the site of injection and occurs only when cytoplasm is injected into the ventral and not into the dorsal side of mutant embryos. The rescuing activity appears to be equally distributed in cleavage stage wild‐type embryos, whereas, in syncytial blastoderm embryos, cytoplasm from the ventral side is about twice as effective as that taken from the dorsal side.

Heat shock induced phenocopies of dominant mutants of the bithorax complex in Drosophila melanogaster

SummaryHeat shock of 37°C applied to Drosophila embryos at blastoderm stage induces phenocopies of some dominant mutants of the bithorax complex. Heat shock might shut down functions of cis regulator genes involved in embryonic pattern formation.

Segmental determination of sensory neurons in Drosophila

The analysis of flies where one segment is transformed into another by mutation or by experimental treatment shows that the central projection of a sensory neuron depends on its segmental determination. The genetic functions that control the segmental determination of neurons and of epidermal cells appear to be distinct, but they have common regulatory features.

Distribution of ring-X chromosomes in the blastoderm of gynandromorphic D. melanogaster

Embryos of the D. melanogaster strain producing gynandromorphs by loss of the ring-X chromosome were treated with vinblastine to obtain blastoderms with all mitoses arrested in metaphase, and with tetracaine to improve the resolution of chromosomes. Ring-X and ringless mitoses were recorded in the major part of the blastoderm in 18 eggs. Limits between females and male areas were very irregular and some embryos had several isolated areas of one type or the other. The proportion of male nuclei varied from 80.8 to 0.4%, indicating that there must have been more than one loss of the ring-X in most of the eggs and that losses occurred as late as the ninth division. When the percentages of male nuclei were compared with theoretical values, all the observed percentages could be accounted for by two losses. In early cleavages the lost ring could be found halfway between ringless mitoses. Examination of chromosomes in the three polar nuclei showed that the ring often remained undivided in meiosis. If the resulting ringless haploid group became a female pronucleus, an XO or YO embryo was produced after fertilization. We propose a hypothesis to explain the two losses.

Maternal and zygotic requirement for thepolyhomeotic complex genetic locus inDrosophila

SummaryThe complex genetic locuspolyhomeotic (ph) is a member of thePolycomb (Pc)-group of genes and as such is required for the normal expression of ANT-C and BX-C genes. It also has probably other functions since amorphicph alleles display a cell death phenotype in the ventral epidermis of 12-h-old embryos. Here it is shown that lethal alleles ofph (amorph and strong hypomorph) show transformation of most of their segments towards AB8. Theph+ product is required autonomously in imaginal cells. The total lack ofph+ function prevents viability of the cuticular derivatives of these cells.ph has a strong maternal effect on segmental identity and epidermal development that can not be rescued by one paternally supplied dose ofph+ in the zygote. These phenotypes differ substantially from those of previously describedPc-group genes. AmongPc-group genes,ph seems to be the only one that is strongly required both maternally and zygotically for normal embryonic development.